Two color pen action

ABSTRACT

A two color pen action has inner and outer concentric cylindrical cams with first and second writing cartridge carriers disposed adjacent to each other therein, the outer cylindrical cam containing two longitudinal cam slots and the inner cylindrical cam containing right and left hand first and second spiral cam slots, the cartridge carriers each having a cam follower extending through a spiral cam slot into a longitudinal cam slot. One cam is fixed to the barrel and the other cam is fixed to the cap of a pen so that relative rotation of the cap and barrel selectively extends a writing cartridge. The inner cylindrical cam extends completely through the outer cam and has an expanding end portion which rotatably locks the cams and thus the cap and barrel together. The first cartridge carrier has a width less than the inside diameter of the inner cam allowing the first cartridge carrier to be inserted into the inner cam until its follower is moved into the first spiral cam slot. The inner cylindrical cam contains a channel leading from one end thereof to the second spiral cam slot so that the second cartridge carrier may be inserted in the inner cam adjacent to the first cartridge carrier with its follower passing through the channel into the second spiral cam slot.

United States Patent [191 Tailer June 11, 1974 1 TWO COLOR PEN ACTION Peter L. Tailer, 323 E. 17th St., New York, N.Y. 10003 22 Filed: Mar. 16,1973 21 Appl. No.: 342,080

Related U.S. Application Data [63] Continuation of Ser. No. 145,699, May 21, 1971,

[76] Inventor:

abandoned, which is a continuation-in-part of Ser.

No. 81,666, Oct. 19, 1970, abandoned.

2,333,889 11/1943 R ueberg 401/78 2,369,080 2/1945 Schulman et a1. i 401/30 3,438,714 4/1969 Seaver 401/78 FORElGN PATENTS OR APPLICATIONS 1,270,645 7/1961 France .Z 401/32 589,077 2/1959 Italy 401/32 Primary Examiner-Lawrence Charles 5 7 ABSTRACT A two color pen action has inner and outer concentric cylindrical cams with first and second writing cartridge carriers disposed adjacent to each other therein, the outer cylindrical cam containing two longitudinal cam slots and the inner cylindrical cam containing right and left hand first and second spiral cam slots, the cartridge carriers each having a cam follower extending through a spiral cam slot into a longitudinal cam slot. One cam is fixed to the barrel and the other cam is fixed to the cap of a pen so that relative rota- 'tion of the cap and barrel selectively extends a writing cartridge. The inner cylindrical cam extends completely through the outer cam and has an expanding end portion which rotatably locks the cams and thus the cap and barrel together.

The first cartridge carrier has a'width less than the inside diameter of the inner cam allowing the first cartridge carrier to be inserted into the inner cam until its follower is moved into the first spiral cam slot. The inner cylindrical cam contains a channel leading from one end thereof to the second spiral cam slot so that the second cartridge carrier may be inserted in theinner cam adjacent to the first cartridge carrier with its follower passing through the channel into the second spiral cam slot.

16 Claims, 21 Drawing Figures slslslooe PATENTEBJUM 1 1 I974 SHEET 2 UP 4 7 0 7 %l L m A T a 3. F m a I m H l I 1| m 5 444 I I i I m v F E 3 4 l 1. E g an. 6 \r l a FIG. 4

TWO COLOR PEN ACTION CROSS-REFERENCE TO RELATED I APPLICATIONS BACKGROUND OF THE INVENTION There is a great need for a two color pen action which is less expensive to manufacture, more rugged in operation, and easily dissambled to change writing cartridges. This invention provides such a pen action.

SUMMARY OF THE INVENTION A two color pen action has inner and outer concentric cylindrical earns, the inner cam containing oppositely disposed right and left hand spiral cam slots and the outer cam containing two oppositely disposed longitudinal cam slots. Two writing cartridge carriers are disposed adjacent to each other within a cylindrical central cavity of the inner cam, each carrier having a cam follower projecting through a spiral cam slot into a longitudinal cam slot. If, as in most cases, the degree of rotation of the cams is to approach 180 degrees, the spiral cam slots must be longitudinally offset, one from the other, and, if the cartridge carriers are substantially identical and designed to hold cartridges of the same length, the followers projecting from each carrier should be longitudinally offset the same distance, relative, to each other, as the spiral cams.

The inner and outer cylindrical cams are fixed in the cap and barrel of a pen so that relative rotation of the cap and barrel will selectively extend a writing cartridge held in a carrier.

The inner cylindrical cam extends completely through the outer cam and has an enlarged, split, spring action end portion which expands beyond the outer cam to hold the cams and thereby the cap and barrel together. This allows the easy disassembly of the pen by merely pulling the cap from the barrel to change writing cartridges. Either the longitudinal or the spiral cam slots may include short circumferential transverse portions, at least at the lower or writing tip end of the travel of each of the followers. This serves to lock the cartridge carriers from longitudinal movement when in their writing positions.

Assembly of the pen action may be accomplished by rounding the sharp edges of the semi-circular section of one of the cartridge carriers and providing a channel from one spiral cam slot to the end of the inner cylindrical cam. This allows the cartridge carrier having rounded edges to be inserted in the inner cam and have its follower seated in the spiral cam slot not having the channel extending therefrom. The second cartridge is then inserted into the inner cam with its follower mov ing down the channel to expand the cam which springs closed as the follower of the second cartridge carrier enters its spiral cam slot. The cylindrical cam containing the longitudinal cam slots may be formed about the inner cam although it preferably has its longitudinal cam slots extend to its end to enable it to be pushed over the inner cam with the followers entering the longitudinal cam slots.

The pen action of this invention enjoys many advantages inherent in its construction and arrangement of parts which will become apparent from the following description and accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal section through the cap and barrel of a two color pen showing the action of this invention therein;

FIG. 2 is a side view of an outer cylindrical cam containing longitudinal cam slots;

FIG. 3 is a longitudinal, vertical section through the outer cylindrical cam of FIG. 2;

FIG. 4 is an end view of an inner cylindrical cam con- FIG. 9 is a longitudinal vertical section through assembled cylindrical cams with two cartridge-carriers disposed therein;

FIG. 10 is a plan view of a sheet metal blank from which a cylindrical cam, as shown in FIGS. 4, 5 and 6, may be rolled and formed;

FIG. 11 is a plan view of a sheet metal blank from which a second embodiment of a cylindrical cam containing spiral cam slots may be rolled and formed to provide a reversed or upside-down action according to this invention;

FIGS. 12-15 are sections taken on lines 12-l2, 13-13, 14-14 and l5-15, respectively, of FIG. 7;

FIG. 16 is a longitudinal vertical section through a pen cap and barrel having a molded plastic pen action therein according to a third embodiment of this invention;

FIG. 17 is a longitudinal vertical section through the pen cap and barrel of FIG. 16 showing the pen action therein, the pen being rotated about a longitudinal axis through I FIG. 18 is a longitudinal vertical section through the cap of the pen of FIG. 16 showing the outer cylindrical cam being removed therefrom;

FIG. 19 is a longitudinal vertical section through the barrel of the pen of FIG. 16 showing the inner cylindrical cam being removed therefrom;

FIG. 20 is a section taken on line 20-20 of FIG. 17; and

FIG. 21 is a section taken on line 2l-21 of FIG. 17

showing cartridge carriers with a modified cross section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a pen, according to a first embodiment of this invention, has a barrel 10 and a cap 11 with a clip 12. As may be seen in FIGS. 2 and 3, a cylindrical cam 13 has an enlarged end 14 and a smaller diameter cam portion 15 containing two longibe assembled to cap 11 in two positions of axial rotation. The cylindrical cam 13 is fixed in barrel by pressing or otherwise securing the enlarged portion 14 in barrel 10. Cam portion 15, as shown in FIG. 1, projects upward from barrel 10.

Referring to FIGS. 1, 4, 5, 6, 9 and 10, a cylindrical cam may be rolled and deformed to the shape shown from the sheet metal stamping 26 of FIG. 10. It may also be formed by die casting, plastic molding, etc. Cam 25 has an enlarged end 27 and a smaller diameter cam portion 28 which is formed to slide within the inside diameter of cam 13. Cam 25 is fixed in cap 11 by pressing its enlarged end 27 in cap 11 or by fixing it in any other suitable manner. Cylindrical cam 25 contains a longitudinal slit 29 if rolled. It also contains two spiral cam slots 30 and 31 spiraling in opposite directions and disposed so that the lower end of one cam slot 30 or 31 is opposite or 180 displaced from the upper end of the other cam slot 30 or 31.

In the preferred embodiment of this invention, the cam slots 30 and 31 are also longitudinally displaced, cam slot 30 being above cam slot 31. This longitudinal displacement is required if the cam slots 30 and 31 approach extending around 180 of the cam portion 28. As shown, the cam slots 30 and 31 extend around 270 of cam portion 28 and thus must be laterally displaced.

Cam slot 31 has a tapering channel 32 leading from the end of cam portion 28 into slot 31. The side of cam portion 28 opposite channel 32 has a longitudinal slot 33 terminating in a stress relief hole 34.

Two locking projections 35 and 36 are formed on the lower end of cam portion 28.-As shown in FIGS. 1 and 9, the cap 11 and barrel 10 are'assembled by sliding cam portion 28 through the entire length of cam 13 so that the locking projections 35 and 36 snap outward below cam 13 rotatably locking cap 11 and barrel 10 together.

Referring now to FIGS. 7, 8 and 12-15, two cartridge carriers 40 and 41 are provided. These carriers are generally semi-circular in section and have larger diameter upper portions 42 which fit within the inner diameter of the central cylindrical cavit of cam 25. Smaller diameter portions 43 project downward from the upper portions 42 to allow the inward springing of the lower end of cam portion 28 when assembling the pen with cartridge carriers 40 and 41 therein. A longitudinal channel 45 has an upper end or termination 46 and receives and holds with a light press fit a ball point pen cartridge 47 or 48 therein as shown in FIG. 1. 1f the cartridge carriers 40 and 41 are molded, they may be formed in a two part mold with interlocking mold projections leaving the channel 45 as shown. Molded cartridge carriers 40 and 41 may include a flat central upper surface 49 in each upper portion 42.

The cartridge carriers 40 and 41 have cam followers 50 and 51 projecting from them. The followers 50 and 51 are longitudinally displaced the same distance as the longitudinal displacement of the spiral cam slots 30 and 31. The side edges 52 of the upper portions 42 are rounded as shown in FIGS. 12-15. I

To assemble the pen, cartridge carrier 40 is inserted upward in cylindrical cam 25, the rounded edges 52 allowing follower 50 to clear the inside diameter until follower 50 falls into cam slot 30. Cartridge carrier 41 is then inserted in cylindrical cam 50, flat to flat with cartridge carrier 40, and follower 51 rides up tapered channel 32 to spring apart the end of channel 32 and enter cam slot 31. Cartridge 47 and 48 may then be placed in the channels 45 of the carriers 40 and 41 and the barrel 10 with cam 13 slipped over cam'portion 28 of cam 25 until locking projections 35 and 36 lock the assembled pen together. As cam 13 is slipped over cam portion 28, the longitudinal cam slots 16 and 17 should be aligned with the followers 50 and 51. This allows the cap 11 and barrel 10 to be assembled in two positions 180 apart. The six locking portions 18-23 of cam slots 16 and 17 are provided to accommodate both assembly positions. v

After assembly, cam followers 50 and 51 project through cam slots 30 and 31 at least partially into cam slots 16 and 17 so that relative rotation of the cap and barrel in different directions alternately projects cartridges 47 and 48. At the limits of rotation, followers 50 and 51 will simultaneously be urged laterally to enter two of the locking portions 18-23. This allows downward force exerted by'the hand of a user on barrel 10 when writing to be taken up by cam 13 which is fixed directly to barrel 10. This prevents writing pressure from tending to force the cap 11 and barrel l0 apart. As cam slots 30 and 31 are shown extending through about 270, the cap and barrel must be rotated to this extent. For a given required longitudinal cartridge travel, a larger rotation provides a less acute cam drive angle.

As an example of a second embodiment of this invention, cam 25 could be fixed in barrel 10 and cam 13 in cap 11. The cartridge carriers would then be inserted to project toward the barrel 10. This would invert the pen action. If cams 25 and 13 were used exactly as shown in an inverted pen action, upward pressure exerted by a follower 50 or 51 on a locking portion 18-23 when the barrel 10 was pressed down to write would tend to cause the locking projections 35 and 36 to release allowing the barrel 10 and cap 11 to start to separate.

As shown in FIG. 11, a blank 26' contains two cam slots 30 and 31 which would form spiral cam slots when the blank 26 was rolled and formed. The cam slots 30 and 31' as shown would extend about 180 and would thus provide a more acute cam drive angle for a given distance of cartridge travel than the slots 30 and 31.0f cam 25. The cam slots 30' and 31 terminate in transverse, circumferential locking portions 60, 61, 62 and 63 so that the cam formed from the blank 26' can be used in an inverted pen action without tending to separate the cap 11 and barrel 10 when writing. When the locking portions 60-63 are provided, the locking portions 18-23 need not be provided in cam With a large enoughdiameter of barrel 10 and cap 11 to accommodate large diameter cams 25 and 13, the spiral cam slots 30 and 31 may extend less than 180 about cam portion 28. Such a design must take into account the required travel distance of the cartridges 47 and 48 to avoid tip interference and the maximum permissible drive angle of the cam slots 30 and 31 determined by the properties of the materials. used. In such a case with cam slots 30 and 31 extending through, for example, the spiral cam slots 30 and 31 and the followers 50and 51 need not be longitudinally offset and the cartridge carriers 40 and 41 may be identical.

Further, while both cams '25 and 13 are shown and described as being fixed, respectively, in the cap 1 1 and the barrel 10, the entire action may be inserted in a long barrellO which only allows portion 27 to project therefrom. Portion 27 may then be formed as a nob, rotation of which selectively extends and retracts car'- tridges 47 and 48.

A third embodiment of this invention is shown in FIGS. 16-20. This embodiment is inverted compared to the first embodiment shown in FIG. 1. In addition, the entire pen is designed to be molded of plastic which requires thicker parts than comparable rolled metal or die cast parts. Barrel 70 has a smaller diameter tip portion 71 to be grasped by a user while writing. A cap 72 is provided with a clip 73.

An outer cam or cap cam 74 contains two longitudinal cam slots 75 terminating in the stress relief apertures 76. Disposed 90 about outer cam 74 from the longitudinal cam slots 75 are the two projecting lugs 77. Cap 72 is molded to contain two transverse apertures 78 which are entered by lugs 77 to lock cam 74 in cap 72. As may be seen in FIG. 18, cam 74 may be removed or replaced by squeezing its projecting end 79 to close the slots 75 anddisengage lugs 77 from the apertures 78.

' An inner or barrel cam 80 has an enlarged end portion 81 which slidably fits in barrel 70. Spring arms 82 have lugs 83 thereon which enter the transverse apertures 84 in barrel 70. Cam 80 may be removed from barrel 70 by forcing the lugs 83 inward with the fingernails as shown in FIG. 19.

' The smaller diameter cam portion 85 of cam 80 extends through the outer cam 74. Cam portion 85 contains right and left hand spiral cam slots 86 and 87. Slot I 87 is entered by the tapered channel 88. Opposite channel 88 there may be formed a channel 89. Cam portion 85 terminates in the enlarged end 90. Channels 88 and 89 allow the end 90 to flex inward and spring outward beyond the end of cam 74 to rotatably lock the cams 74 and 80, and thereby the cap 72 and barrel 70, together when the pen is assembled. I

Cartridges 91 and 92 are inserted in the cartridge carriers 93 and 94 having the followers 95 and 96 projecting therefrom. In assembly, the rounded corners 97 allow cartridge carrier 93 to be inserted in the central cylindrical cavityof cam 80 until follower 95 may be moved outward to enter cam slot 86. Cartridge carrier 94 is then slid downward, as shown, with its follower 96 moving down in channel 88 until it enters spiral cam tapered to allow end 90 to be squeezed inward by cam 74 during assembly. The pen may be molded of transparent plastic to render the mechanism visible.

As shown in FIG. 20, longitudinal indentations 98 are formed along the sides of the carriers 93 and 94 to limit longitudinal warping during molding.

As shown in FIG. 21, modified cartridge carriers 93' and 94 are used. Carrier 93' has the cut away edges 99 which allow it to be inserted easily in cam 80 until its follower enters cam slot 86. Cartridge carrier 94' extends about more than l80of the central cavity of cam 80. The inwardly disposed surfaces of theadjacent carriers 93 and 94 may be in any configuration provided the first inserted carrier does not bind across the diameter of the central cylindrical cavity of cam 80. This allows thefirst inserted carrier 93 and its projecting follower to be easily inserted in cam 80. Thus one cartridge carrier must have a lateral maximum dimension or width which is less than the diameter of the central cylindrical cavity of cam 80.

Although cam 80 is shown fixed in barrel and cam 74 in cap 72, the proportions of barrel 70 and cap 72 could be changed so that cam fit in a cap 72 and cam 74 in a barrel 70. The cartridge carriers would be redesigned to be reversed and locking portions would be provided in cam 74. The pen action could thus be inverted. I

While this invention has been shown and described in the best forms known, it will nevertheless be understood that these are purely exemplary and that modifications may be made without departing from the spirit of the invention.

I claim: I r

1. A pen action for a two cartridge pen comprising, in combination, two cylindrical cams mounted for relative rotation having inner and outer cam'portions with said inner cam portion extending within said outer cam portion andwith said inner cam portion containing a cylindrical cavity, said outer cam portion containing longitudinal cam slots extending to the end thereof and said inner cam portion containing first and second right and left hand spiral cam slots, and first and second cartridge carriers having centrally disposed surfaces contacting each other within said central cavity of said inner cam portion, said inner cam portion having a channel extending from the end of said inner cam portion to said second spiral cam slot, said cartridge carri ers each having a cam follower extending through a spiral cam slot of said inner cam portion into a longitudi- .nal cam slot of said outer cam portion, relative rotation of said cylindrical cams alternately extending and retracting said cartridge carriers, said first cartridge carrier having a maximum width less than the inside diameter of said central cylindrical cavity so that said first cartridge carrier and it cam follower may be inserted in said central cavity until the cam follower enters and moves outward into said first spiral cam slot, said second cartridge carrier being slid into said central cavity adjacent to said first cartridge carrier with its cam follower riding down said channel to enter said second spiral cam slot.

2. The combination according to claim 1 wherein said inner cam portion has at least one locking projection thereon springing outward rotatably locking said cylindrical cams together.

3. The combination according to claim 1 wherein said inner cam portion has, one end extending completely through said cylindrical cam having said outer cam portion, said end of said inner cam portion having at least one locking projection thereon and said channel allowing said at least one locking projection to spring outward beyond said cylindrical cam having said outer cam portion rotatably locking said cylindrical cams together.

4. The combination according to claim 3 wherein said cartridge carriers have smaller diameter portions extending within said end of said inner cam portion allowing said locking projection to spring inward closing at least partially said channel during assembly of the pen action. a

5. The combination according to claim 1 wherein said first and second spiral cam slots are longitudinally offset, said second spiral cam slot being nearest said end of said inner cam portion, said cam followers on said cartridge carriers being offset the same distance as said spiral cam slots.

6. A two cartridge pen comprising, in combination, a barrel, a cap, a first cylindrical cam having an inner cam portion and a fixed end portion, said first cylindrical cam containing a central cylindrical cavity, a second cylindrical cam having an outer cam portion and an end portion, said inner cam portion being adapted to enter said outer cam portion, said end portions being .fixed in said cap and said barrel, said outer cam portion containing longitudinal cam slotsextending to the end thereof and said inner cam portion containing first and second right and left hand spiral cam slots, first and second cartridge carriers disposed side by side in said central cylindrical cavity of said inner cam portion, said inner cam. portion having a channel extending from the end of said inner cam portion to said second spiral cam slot, each of said cartridge carriers having a cam follower extending through a spiral cam slot in said inner cam portion into a longitudinal cam slot in said outer cam portion, and a writing cartridge fixed to each cartridge carrier so that relative rotation of said cap and barrel selectively extends and retracts each cartridge, said first cartridge carrier having a maximum width less than the inside diameter of said central cavity so that during assembly said first cartridge carrier and its cam follower may be inserted in said centralcavity until the cam follower moves outward to enter said first spiral cam slot, said second cartridge carrier being slid into said central cavity adjacent to said first cartridge carrier until its follower rides down said channel to enter said second spiral cam slot.

7. The combination according to claim 6 wherein said inner cam portion has a free end opposite its fixed end portion, said free end extending through said second cylindrical cam to project beyond the fixed end of portion of said second cylindrical cam, said free end of said inner cam portion having at least one locking element thereon so that said locking element springs outward beyond said second cylindrical cam rotatably locking said cap and barrel together.

8. The combination according to claim 7 wherein said spiral cam slots extend at least 180 about said inner cam portion, said spiral cam slots being longitudinally offset from each other and said cam followers of said .cartridge carriers being longitudinally offset from each other the same distance as said spiral cam slots.

, 9. The combination according to claim 7 wherein said fixed end of said first cylindrical cam is fixed in and fixing said first cylindrical cam therein.

said cap and said fixed end of said second cylindrical cam is fixed in said barrel, said longitudinal cam slots in said second cylindrical cam including lateral, circumferential locking portions at the extremes of cam follower travel in said longitudinal cam slots.

10. The combination according to claim 7 wherein said fixed end of said first cylindrical cam is fixed in said barrel and said fixed'end of said second cylindrical cam is fixed in said cap, at least the ends of said spiral cam slots disposed towards said barrel forming short lateral, circumferential locking portions.

llQThe combination according to claim 7 wherein said second cam projects from said cap and said cap contains at least one transverse aperture, and with the addition of at least one lug projecting from the outer cam portion of said second cam between said longitudinal cam slots so that the projecting end of said second cam may be squeezed to insert and disengage said at leastone lug from said at least one transverse aperture.

12. The combination according to claim 11 wherein said cap contains two oppositely disposed transverse apertures and said second cam has two oppositely disposed lugs projecting therefrom to enter said transverse apertures.

13. The combination according to'claim 12 wherein said barrel contains two oppositely disposed transverse apertures and said end portion of said first cylindrical cam has two spring arms with lugs thereon, said lugs on said arms entering the transverse apertures of said barrel fixing said first cylindrical cam therein.

14. The combination according to claim 7 wherein said second cam projects from said barrel and said barrel contains at least one transverse aperture, and with the addition of at least one lug projecting from the outer cam portion of said second cam between said longitudinal cam slots so that the projecting end of said second cam may be squeezed to insert and disengage said at least one lug from said at least one transverse aperture. t

15. The combination according to claim 14 wherein said barrel contains two oppositely disposed transverse apertures and said second cam has two oppositely disposed lugs projecting therefrom to enter said transverse apertures.

16. The combination according to claim 15 wherein said cap contains two oppositely disposed transverse apertures and said end portion of said first cylindrical cam has two spring arms with lugs thereon, said lugs on said arms entering the transverse apertures of said cap 

1. A pen action for a two cartridge pen comprising, in combination, two cylindrical cams mounted for relative rotation having inner and outer cam portions with said inner cam portion extending within said outer cam portion and with said inner cam portion containing a cylindrical cavity, said outer cam portion containing longitudinal cam slots extending to the end thereof and said inner cam portion containing first and second right and left hand spiral cam slots, and first and second cartridge carriers having centrally disposed surfaces contacting each other within said central cavity of said inner cam portion, said inner cam portion having a channel extending from the end of said inner cam portion to said second spiral cam slot, said cartridge carriers each having a cam follower extending through a spiral cam slot of said inner cam portion into a longitudinal cam slot of said outer cam portion, relative rotation of said cylindrical cams alternately extending and retracting said cartridge carriers, said first cartridge carrier having a maximum width less than the inside diameter of said central cylindrical cavity so that said first cartridge carrier and it cam follower may be inserted in said central cavity until the cam follower enters and moves outward into said first spiral cam slot, said second cartridge carrier being slid into said central cavity adjacent to said first cartridge carrier with its cam follower riding down said channel to enter said second spiral cam slot.
 2. The combination according to claim 1 wherein said inner cam portion has at least one locking projection thereon springing outward rotatably locking said cylindrical cams together.
 3. The combination according to claim 1 wherein said inner cam portion has one end extending completely through said cylindrical cam having said outer cam portion, said end of said inner cam portion having at least one locking projection thereon and said channel allowing said at least one locking projection to spring outward beyond said cylindrical cam having said outer cam portion rotatably locking said cylindrical cams together.
 4. The combination according to claim 3 wherein said cartridge carriers have smaller diameter portions extending within said end of said inner cam portion allowing said locking projection to spring inward closing at least partially said channel during assembly of the pen action.
 5. The combination according to claim 1 wherein said first and second spiral cam slots are longitudinally offset, said second spiral cam slot being nearest said end of said inner cam portion, said cam followers on said cartridge carriers being offset the same distance as said spiral cam slots.
 6. A two cartridge pen comprising, in combination, a barrel, a cap, a first cylindrical cam having an inner cam portion and a fixed end portion, said first cylindrical cam containing a central cylindrical cavity, a second cylindrical cam having an outer cam portion and an end portion, said inner cam portion being adapted to enter said outer cam portion, said end portions being fixed in said cap and said barrel, said outer cam portion containing longitudinal cam slots extending to the end thereof and said inner cam portion containing first and second right and left hand spiral cam slots, first and second cartridge carriers disposed side by side in said central cylindrical cavity of said inner cam portion, said inner cam portion having a channel extending from the end of said inner cam portion to said second spiral cam slot, each of said cartridge carriers having a cam follower extending through a spiral cam slot in said inner cam portion into a longitudinal cam slot in said outer cam portion, and a writing cartridge fixed to each cartridge carrier so that relative rotation of said cap and barrel selectively extends and retracts each cartridge, said first cartridge carrier having a maximum width less than the inside diameter of said central cavity so that during assembly said first cartridge carrier and its cam follower may be inserted in said central cavity until the cam follower moves outward to enter said first spiral cam slot, said second cartridge carrier being slid into said central cavity adjacent to said first cartridge carrier until its follower rides down said channel to enter said second spiral cam slot.
 7. The combination according to claim 6 wherein said inner cam portion has a free end opposite its fixed end portion, said free end extending through said second cylindrical cam to project beyond the fixed end of portion of said second cylindrical cam, said free end of said inner cam portion having at least one locking element thereon so that said locking element springs outward beyond said second cylindrical cam rotatably locking said cap and barrel together.
 8. The combination according to claim 7 wherein said spiral cam slots extend at least 180* about said inner cam portion, said spiral cam slots being longitudinally offset from each other and said cam followers of said cartridge carriers being longitudinally offset from each other the same distance as said spiral cam slots.
 9. The combination according to claim 7 wherein said fixed end of said first cylindrical cam is fixed in said cap and said fixed end of said second cylindrical cam is fixed in said barrel, said longitudinal cam slots in said second cylindrical cam including lateral, circumferential locking portions at the extremes of cam follower travel in said longitudinal cam slots.
 10. The combination according to claim 7 wherein said fixed end of said first cylindrical cam is fixed in said barrel and said fixed end of said second cylindrical cam is fixed in said cap, at least the ends of said spiral cam slots disposed towards said barrel forming short lateral, circumferential locking portions.
 11. The combination according to claim 7 wherein said second cam projects from said cap and said cap contains at least one transverse aperture, and with the addition of at least one lug projecting from the outer cam portion of said second cam between said longitudinal cam slots so that the projecting end of said second cam may be squeezed to insert and disengage said at least one lug from said at least one transverse aperture.
 12. The combination according to claim 11 wherein said cap contains two oppositely disposed transverse apertures and said second cam has two oppositely disposed lugs projecting therefrom to enter said transverse apertures.
 13. The combination according to claim 12 wherein said barrel contains two oppositely disposed transverse apertures and said end portion of said first cylindrical cam has two spring arms with lugs thereon, said lugs on said arms entering the transverse apertures of said barrel fixing said first cylindrical cam therein.
 14. The combination according to claim 7 wherein said second cam projects from said barrel and said barrel contains at least one transverse aperture, and with the addition of at least one lug projecting from the outer cam portion of said second cam between said longitudinal cam slots so that the projecting end of said second cam may be squeezed to insert and disengage said at least one lug from said at least one transverse aperture.
 15. The combination according to claim 14 wherein said barrel contains two oppositely disposed transverse apertures and said second cam has two oppositely disposed lugs projecting therefrom to enter said transverse apertures.
 16. The combination according to claim 15 wherein said cap contains two oppositely disposed transverse apertures and said end portion of said first cylindrical cam has two spring arms with lugs thereon, said lugs on said arms entering the transverse apertures of said cap and fixing said first cylindrical cam therein. 